Tubular structures are used in a variety of applications such as, for example and without limitation, liquid and gas storage tanks, rocket motor casings, flywheel rotors and structural support members, to name only a few. In some applications, it may be desirable to fabricate these tubular structures using fiber reinforced composites in order to achieve lower manufacturing costs and/or higher strength per unit rate ratios.
In the past, composite tubular structures have been fabricated by winding tows of fibers over a rotating mandrel. The fibers may be pre-impregnated with a polymer resin, or the resin may be added to the fibers during mandrel winding. This prior process, while effective, may not be well suited for applications where both high strength and low manufacturing costs are desired. For example, in order to maintain the fiber volume fraction of the resulting composite relatively high, the fibers must be maintained in tension during the winding process, requiring long lengths of fiber which may increase the cost of fiber feed stock. Shorter fibers could be used to reduce the cost of the feed stock, however a high fiber volume fraction may be difficult to obtain using shorter fibers, in part because it is difficult to apply the necessary pressure to the fibers during the winding process, and the centrifugal force generated by the mandrel tends to throw short fibers outwardly away from the mandrel.
Other processes have been used to produce composite tubular structures, particularly for high volume production, including high speed extrusion, injection molding, and pultrusion. However, these manufacturing techniques may result in largely random orientations which may be undesirable for composite strength.
In addition to the need for a relatively high fiber volume and directional fiber orientation, higher performance levels of composite tubular structures may require that the reinforcing fibers in the final product have a sufficiently high aspect ratio, i.e. length-to-diameter ratio. The need for high fiber volumes, controlled fiber orientation and the use of fibers having high aspect ratios may make the use of conventional high speed production techniques impractical. For example, the requirements of high fiber volume and aspect ratios may result in fiber breakage during processing from fiber-polymer interaction, fiber-fiber interaction, and fiber contact with surfaces of processing equipment. In general, the fiber length decreases as the fiber volume fraction increases, making it difficult to obtain sufficiently long fibers to obtain the highest performance with low cost, high value production processes. Finally, as previously mentioned, known production processes tend to result in a relatively high degree of randomness in short fiber orientation, which may undesirably affect performance of the composite, compared to the performance that may be obtained using continuous fibers and controlled orientation provided by wrapping continuous fibers under tension around a spinning mandrel.
Accordingly, there is a need for a cost-effective method and apparatus for making fiber reinforced tubular structures that use short fibers having optimized aspect ratios in which high fiber fractions and fiber alignment are achieved to meet high performance requirements.